Isolation Effects of Aquatic Habitat Fragmentation on Greenside (Etheostoma blennioides) and Fantail (Etheostoma flabellare) Darters

Aquatic habitat fragmentation is defined as the lack of connectivity between upstream and downstream populations and can occur along two gradients, the longitudinal gradient and the lateral gradient. The longitudinal gradient focuses on objects within the stream and is concerned with the disruption of stream flow, which is normally brought about by the presence of dams and bridges. Fragmentation along the lateral gradient, on the other hand, examines factors outside of the stream and is related to any loss or modification of the surrounding riparian zone. In Illinois within the last 100 years, stream habitats have become increasingly fragmented by dams, pollution, and the loss of riparian zones, which have been brought about by an increase in channelization, wetland drainage, groundwater exploitation, and most importantly, agriculture. Organisms subjected to aquatic fragmentation may form isolated populations, with limited dispersal possibilities, leading to a reduction in gene flow and/or loss of genetic variation. Intolerant species, such as darters, may become isolated in distinct patches of good habitat leading to genetic differentiation between populations. To determine if aquatic habitat fragmentation has had an isolating effect on darters within a stream system, the following objectives were established: 1) estimate population size of three isolated populations of greenside and fantail darters, 2) determine the dispersal rate of individuals between isolated good patches, and 3) assess the degree of genetic isolation of darters between good patches. Three hundred sixty-two greenside darters (Etheostoma blennioides) and 169 fantail darters (Etheostoma flabellare) from three good sites along Polecat Creek in Coles County, Illinois, were injected with an elastomer marking system and released back into the stream. Ten recapture attempts yielded only one marked fantail, which precluded any other data being gathered for this species; however, overall greenside recapture rate was 23% (site 1 = 20%, site 2 = 6%, site 3 = 43%). Minimal greenside darter movement was found between riffles within sites, but no movement took place between sites. Allozyme analyses were performed on greenside darters to determine genetic differences between darter populations in distinct good patches. Mean heterozygosity was high for all three populations (0.21, 0.22, and 0.19, respectively) with no significant difference among sites. A modified Rogers\u27 distance revealed that populations 1 and 2 were genetically similar, whereas population 3 was more genetically distinct. Thus, the isolating effects of habitat fragmentation have led to a decrease in overall population size and a lack of dispersal among sites. If population isolation is maintained and, at the same time, population size remains low and gene flow does not occur, we can predict that all three greenside darter populations will likely experience a loss of heterozygosity and/or rare alleles due to increased genetic drift and/or inbreeding, leading to possible extinction of the population